bigquery: support for JSON type

[rebeccajan]

Is your feature request related to a problem? Please describe.
It would be possible to convert a bigquery.Schema to a storagepb.TableSchema using the adapt package functionality if the storagepb library supported JSON data types.

Given a bigquery.schema with a field Payload and a data type JSON, the below code:
schema, err := adapt.BQSchemaToStorageTableSchema(schema)
Throws the error:
"could not convert field (Payload) due to unknown type value: JSON"

Describe the solution you'd like
Support for the JSON data type in the bigquery client libraries i.e. storagepb.TableSchema, this functionality exists for Java and Node libraries.

Describe alternatives you've considered
Using a STRING field which takes a stringified JSON instead of an actual JSON - but this is not ideal.

Additional context
Add any other context or screenshots about the feature request here.
N/A